1. Technological Field
This technical disclosure pertains generally to magnetic direction sensors, and more particularly to overcoming offset issues with Lorentz force magnetic sensors.
2. Background Discussion
Ongoing efforts are being made toward improving both the sensitivity and direction resolution of magnetic sensors. The Lorentz force magnetic sensor is an emerging technology for magnetic sensors, and Lorentz force sensors with sensitivity and resolution comparable to, or better than, that of Hall-effect sensors have been reported recently. Compared to anisotropic magnetoresistive (AMR) and Hall-effect sensors that are commonly used in portable electronics, the Lorentz force sensor has the advantage that it is free of magnetic material, whereby it is potentially CMOS compatible, while its lack of magnetic material result in its being free of magnetic hysteresis. A Lorentz force sensor can also be co-fabricated with other MEMS (micro-electromechanical systems) inertial sensors, such as accelerometers and gyroscopes.
Offset is another key parameter that influences the performance of magnetic sensors. Offset reduces the dynamic range of the system and also results in drift error, which directly transfers to heading error when the sensor is used as a compass. It should be appreciated that there are three general categories of compasses: (1) cardinal points, (2) heading/orienteering, and (3) navigation. For cardinal points, the compass is used to provide eight major directions (N, NE, E, SE, S, SW, W, NW). The required resolution for this application is 10 μT with a heading accuracy of at least 22.5°. The second category of heading/orienteering is the type of compasses mainly used in portable devices, such as smart phones and tablets. The required resolution is 500 nT and heading accuracy is 1°. A 1 μT drift error in the sensor output may result in more than 1° error in azimuth determination. In the last category, compasses for navigation, provide a resolution of better than 50 nT, and with 0.1° heading accuracy. Flux-gate magnetometers and AMR magnetometers are the common sensors in this category due to their lower noise. Methods of reducing the offset and drift error for Hall-effect sensors and AMR sensors have been reported.
However, offset and offset instability in the Lorentz force sensor has not been well-studied as yet. Kyynäräinen et al. reported a stability measurement conducted over 120 hours and observed a 1% (±10 000 ppm) variation in the sensor's output. In previous work of the Applicant, Allan deviation was utilized as a measure of the random variation of offset as a function of averaging time. Both of the above works indicated that despite the low short-term noise of the Lorentz force sensor, long-term drift is a problem when using this sensor as an electronic compass.
Accordingly, apparatus and methods are described in the present disclosure for overcoming drift when using Lorentz force sensors.